A white light source is generally provided by mixing light sources of different wavelengths. For example, a conventional white light source can be realized by mixing red light, green light and blue light with a suitable intensity ratio. Alternatively, the white light source can be realized by mixing yellow light and blue light with a suitable intensity ratio. The conventional method for manufacturing a white light source are summarized in the following.
In a first prior art white light source, three LED dies based on InGaAlP, InGaN and Gap are packaged into a lamp and emit red light, blue light and green light, respectively. The light emitted from the lamp can be mixed by a lens to provide white light.
In a second prior art white light source, two LED dies based on InGaN (or AlInGaP) and Gap emit blue light and yellowish-green light. The blue light and yellowish-green light are mixed to provide white light.
A third prior art white light source is proposed by Nichia Chemical Co. An InGaN based blue LED and a yellow YAG phosphor are used to provide the white light source. This white light source requires a single color LED to provide white light. Moreover, the phosphor is a mature art and commercially available.
A fourth prior art white light source utilizes UV LED to excite phosphors of blue, green and red color for mixing into white light.
In first and second prior art white light source, LEDs for multiple colors are required. The color of the white light source is distorted if one of the LEDs malfunctions. Moreover, the light emission efficiency is not satisfactory. The second and third prior art white light source employs complementary colors to achieve white light. However, the white light produced in this way is not uniform in spectral distribution (especially in 400 nm–700 nm) as is natural white light such as sunlight. The white light thus produced has relatively poor chroma, which is, even though indistinguishable to human eyes, differentiable to instruments such as a camera. Therefore, the color rendering property and reproducing ability are not satisfactory and this white light source is used mainly for lighting.
In fourth prior art white light source, it is preferable to use three phosphors for emitting three different colors to enhance the color rendering property thereof. However, the phosphors should be prudently chosen to have absorption band matched with the wavelength of the exciting radiation. Moreover, the phosphors should have compatible absorption coefficients and quantum efficiency to provide white light of high quality. These requirements place a strict constraint on the materials of the phosphors.